1. Field of the Invention
The present invention relates to a communication terminal device that controls control functions in a reception environment, and to a reception environment reporting method.
2. Description of the Related Art
With the rapid Internet spread in recent years, diversification of information, larger volume of information, and development of the next-generation Internet, research and development of the next-generation radio access network system that realizes a fast radio transmission system in mobile communication. In such a next-generation fast radio access network such as High Speed Downlink Packet Access (HSDPA), namely the fourth-generation communications system, Adaptive Modulation and channel Coding (AMC) that varies the throughput according to the reception environment of a mobile device is applied. Hence, the report on the reception environment of the mobile device plays an important role.
As indexes of the reception environment of the mobile device, there are Signal-to-Interference power Ratio (SIR), Received Signal Code Power (RSCP), Channel Quality Indicator (CQI), and the like. In particular, CQI is widely used in AMC, as described in “3rd Generation Partnership Project: Technical Specification Group Radio Access Network: Physical Layer Procedures (FDD) (Release 5)” in 3GPP TS 25.214 V5.11.0 (2005-06) (hereinafter, referred to as non-Patent Document 1).
Referring now to FIG. 10, the configuration of a mobile device 1 adapted to a conventional AMC is shown. An RX radio section 11 of the mobile device 1 amplifies signals transmitted from a base transceiver station 2 and converts the signals into digital ones, so as to perform a reverse diffusion process on each Common Pilot Channel (CPICH), a control signal, and a data signal. A RAKE synthesizer 13 performs RAKE synthesis on the signal of each path subsequent to the reverse diffusion process. A rate information detector 17 detects rate information from the control signal output from the RAKE synthesizer 13. A decoder 18 decodes the data signal output from the RAKE synthesizer 13 based upon the rate information, and obtains the received user data.
An SIR measuring section 14 measures an SIR from CPICH output from the RAKE synthesizer 13. A CQI converter 15 converts the SIR measured by the SIR measuring section 14 into a CQI. In this process, the CQI value of the mobile device 1 is set in such a manner that a Block Error Rate (BLER) of the SIR value should not exceed 10% of the target value defined in a CQI mapping table specified by 3GPP, under the current environment. Generally, an SIR-CQI conversion table that satisfies the above condition is prepared beforehand, so that the CQI value is calculated from the SIR value by use of the conversion table. A CQI transmitter 16 transmits the CQI value that has been calculated by the CQI converter 15 to a base transceiver station.
FIG. 11 is a view showing a communication control procedure by means of the AMC. The mobile device 1 measures the SIR value from the CPICH value of the signal that has been received from a base transceiver station 2 (step S401), converts the measured SIR value into the CQI value and transmits the CQI value to the base transceiver station 2, so as to make a report of the reception environment (step S402).
The base transceiver station 2, in response to the CQI value received from the mobile device 1, selects the optimal transmission rate (hereinafter, simply referred to as TBS, an abbreviation for Transport Block Size), and transmits signals to the mobile device 1 by means of the TBS (step S403). Thus, the optimal throughput is achieved.
Regarding the SIR value measured by the mobile device, the measurement accuracy is significantly degraded depending on the change in the environment such as the mobile speed of the mobile device, thereby resulting in the degradation of the throughput. For instance, in a case where the mobile speed of the mobile device 1 is low (for example, equal to or lower than 30 km/h), the variance of the received signal is relatively small due to the low-speed phasing and the SIR value is measured with substantial accuracy. In a case where the mobile speed is high, the wider signal variance occurs. This causes the interference component to be calculated greater. Therefore, the SIR value is measured to be smaller than the actual environment, so the mobile device 1 may make a report of a smaller CQI value.
If the mobile device 1 makes a report of a value greater than an appropriate CQI value, a signal with a greater TBS will be transmitted from the base transceiver station 2, thereby leading to the increased block error rate. Consequently, the throughput will be degraded. Conversely, if the mobile device 1 makes a report of a smaller CQI value, a signal with a smaller TBS will be transmitted from the base transceiver station 2, thereby decreasing the block error rate. However, also in this case, the throughput will be degraded.
In order to prevent the degradation of the throughput caused by the report of an inappropriate CQI value, the CQI value is corrected by the packet error rate in the third-generation communications system, as described in JP 2007-521750 A. Meanwhile, in the fourth-generation communications system to which the AMC is applied, there is a connection between the CQI value reported to the base transceiver station, and the TBS and the block error rate of the signal transmitted from the base transceiver station. Therefore, there is a technique of storing a conversion table in which 30 SIR threshold values and CQI values are related to a mobile device, updating the SIR threshold values of the conversion table when the “the number of normal reception times/the number of abnormal reception times” of the data block exceeds a given number of times, and adjusting the CQI value to be converted by use of the conversion table from the measured SIR value, as described in JP 2005-64963 A.
According to Patent Document 2, however, 30 SIR threshold values are increased or decreased at a time by the same value at the time of updating the conversion table. Such an update method of the conversion table does not always reflect the reception environment in detail. In addition, since the conversion table is updated when the “the number of normal reception times/the number of abnormal reception times” of the data block exceeds a given number of times, the update frequency of the conversion table is reduced depending on the settings of the given number of times. Accordingly, the reception environment cannot be reflected on the conversion table in a timely manner and the reception environment cannot be reported with accuracy. Furthermore, whenever the conversion table is updated, updating of 30 SIR threshold values is a complicated and wasteful process.